1. Field of the Invention
The present invention is directed to a method of electrically testing an electrical component containing a plurality of networks with at least one node. More particularly, the invention uses segmented, charge limiting testing to charge the nodes and detect shorted and disconnected nodes while preventing accumulated charges in the networks from making uncharged nodes appear charged.
2. Description of Related Art
Advanced substrate fabrication strategies require judicious use of in-process final test and inspection tools. Automatic test and inspection techniques continually need to be improved to match increasing complexity of advanced electrical components. In addition, test and inspection are often keys in reducing costs and increasing functionality of electrical networks in electrical components.
High packaging densities require finer line and pad size and more wiring levels of interconnection networks in order for a single component to have 10,000 chip-to-board interconnect bonding pads. To reduce costs and increase yields it may be necessary to verify the correctness of each process step where many such steps are used to produce the final component. Electrical testing of sub-assemblies as well as the completed components becomes increasingly difficult as the density of the networks increases and the dimensions of pads decrease. As a result, conventional methods of mechanical probing such as flying probe or bed-of-nails testers are not likely to meet reliability and cost effectiveness standards. New techniques are needed to replace expensive, delicate and short lived mechanical probes that may damage the component being tested.
One such technique is voltage contrast electron beam testing. Electron beams can charge networks of pads within unpopulated substrates as well as dynamically sense (read) the electrical potential of the pads. This discrimination between charged and uncharged pads allows for detection of shorts and disconnects (opens). Voltage contrast electron beam testing requires that the beam be accurately vectored to probe the nodes where it must charge the nodes or measure the node voltages uniformly over large areas. Further, before testing and between complete test sequences an electron flood gun must uniformly remove the charges from the component. The physics of electron beam interactions with various substrate materials governs the ability to charge, read voltage, and erase charge in the materials. Beam parameters must be determined for each set of conductor/insulator substrate materials. Charge retention by the conductor networks must exceed testing times and inadvertent contamination can cause unacceptable leakage resistances. Therefore, substrate material condition, cleaning methods, and conditioning requirements must be specified. Advantageously, electron beams can be positioned by rapid electromagnetic or electrostatic deflection systems based on CAD data alone. Furthermore, electron beams have been used for pad sizes below 100 microns and pad numbers of several tens of thousands. For instance, in "A Dynamic Single E-Beam Short/Open Testing Technique," Scanning Electron Microscopy/1985/III (pages 991-999) Brunner et al. report that on a module containing 30,000 pads the charge and read times summed up to less than one minute. Thus, electron beams are an effective tool for testing a wide variety of printed circuit boards, multichip modules, substrates and other electrical components. Voltage contrast electron beam systems are disclosed, for example, in U.S. Pat. Nos. 4,829,243 to Woodard et al. and 4,843,329 to Beha et al.
For a more complete understanding of the present invention, it is necessary to discuss a few related test algorithms. A one-pass test method (nodes tested once for shorts and once for opens) well suited for electron beam testing is taught by R. Schmid et al. in "Design and application of an e-beam test system for microwiring substrates,"34th Electron, Ion and Photon Beams Conference, R2, May 29, 1990. Another one-pass test method is disclosed by Myers et al. in U.S. Ser. No. 07/631,111 filed Dec. 20, 1990. Such one-pass algorithms are generally performed by charging a node in a first network and testing the other nodes in the first network for being uncharged (disconnected or open), testing each node in a second network for being charged (shorted) and then charging one node in the second network and testing the remaining nodes in the second network for being uncharged, and repeating this procedure for the remaining networks until either all the nodes have been tested or a defect has been found. Such one-pass tests are relatively fast but ascertain only the existence of opens or shorts in the component.
The necessity of determining the location of any defect depends on the purpose of conducting the test. Various test purposes include (1) determining whether the part is good or bad; (2) identifying bad networks; and (3) a complete functional description of network interconnection. The one-pass open and short testing procedure described above can be employed to discriminate good substrates from bad substrates, which may be sufficient in many cases as the occurrence of defects may be infrequent. The one-pass test which effectively is a simple open and short test may be sufficient particularly where the substrate needs to be labelled good or bad but fault location is not required. The one-pass test may be advantageous since the relatively few decisions, measurements, and charge excitations permit rapid testing. In addition, the one-pass test can be discontinued after the first defect is detected. However, it may be desirable to more fully test the networks to detect more of the defects therein in order to provide further fault information. Myers et al. disclose a two-pass test to provide such information. The first pass comprises the above-mentioned one-pass test with a few additional steps. The second pass algorithms are relatively complicated and shall not be described herein. To summarize, the two-pass tests determine which of the networks are defective; the more sophisticated two-pass test also determines precisely how the faulty nodes or networks are connected and thus details all functional faults.
Unfortunately, performing such tests with electron beam and voltage contrast equipment to detect opens and shorts may cause charge buildup in high density multichip modules. As more and more electrical networks are charged to find opens and shorts, the electrical potential of other networks rises. Depending on the density and design of the multichip module, it is possible for the potential of nodes in the networks which have not been charged to appear as shorted when, in fact, they are not. This drawback is hereinafter referred to as the "accumulated charge effect." Needless to say, there is a need for a charge limiting test algorithm which overcomes the accumulated charge effect.